Activation of native TRPC1/C5/C6 channels by endothelin-1 is mediated by both PIP3 and PIP2 in rabbit coronary artery myocytes

TRPC1 protein channel is major regulator of epidermal growth factor receptor signaling

TRP channels have been associated with cell proliferation and aggressiveness in several cancers. In particular, TRPC1 regulates cell proliferation and motility, two processes underlying cancer progression. We and others have described the mechanisms of TRPC1-dependent cell migration. However, the involvement of TRPC1 in cell proliferation remains unexplained. In this study, we show that siRNA-mediated TRPC1 depletion in non small cell lung carcinoma cell lines induced G(0)/G(1) cell cycle arrest resulting in dramatic decrease in cell growth. The expression of cyclins D1 and D3 was reduced after TRPC1 knockdown, pointing out the role of TRPC1 in G(1)/S transition. This was associated with a decreased phosphorylation and activation of EGFR and with a subsequent disruption of PI3K/Akt and MAPK downstream pathways. Stimulation of EGFR by its natural ligand, EGF, induced Ca(2+) release from the endoplasmic reticulum and Ca(2+) entry through TRPC1. Ca(2+) entry through TRPC1 conversely activated EGFR, suggesting that TRPC1 is a component of a Ca(2+)-dependent amplification of EGF-dependent cell proliferation.

Integration of transient receptor potential canonical channels with lipids

Transient receptor potential canonical (TRPC) channels are the canonical (C) subset of the TRP proteins, which are widely expressed in mammalian cells. They are thought to be primarily involved in determining calcium and sodium entry and have wide-ranging functions that include regulation of cell proliferation, motility and contraction. The channels are modulated by a multiplicity of factors, putatively existing as integrators in the plasma membrane. This review considers the sensitivities of TRPC channels to lipids that include diacylglycerols, phosphatidylinositol bisphosphate, lysophospholipids, oxidized phospholipids, arachidonic acid and its metabolites, sphingosine-1-phosphate, cholesterol and some steroidal derivatives and other lipid factors such as gangliosides. Promiscuous and selective lipid sensing have been detected. There appear to be close working relationships with lipids of the phospholipase C and A₂ enzyme systems, which may enable integration with receptor signalling and membrane stretch. There are differences in the properties of each TRPC channel that are further complicated by TRPC heteromultimerization. The lipids modulate activity of the channels or insertion in the plasma membrane. Lipid microenvironments and intermediate sensing proteins have been described that include caveolae, G protein signalling, SEC14-like and spectrin-type domains 1 (SESTD1) and podocin. The data suggest that lipid sensing is an important aspect of TRPC channel biology enabling integration with other signalling systems.

Phosphoinositide sensitivity of ion channels, a functional perspective

Phosphoinositides, especially phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] are required for the activity of many different ion channels. This chapter will highlight various aspects of this paradigm, by discussing current knowledge on four different ion channel families: inwardly rectifying K(+) (Kir) channels, KCNQ voltage gated K(+) channels, voltage gated Ca(2+) (VGCC) channels and Transient Receptor Potential (TRP) channels. Our main focus is to discuss functional aspects of this regulation, i.e. how changes in the concentration of PtdIns(4,5)P(2) in the plasma membrane upon phospholipase C activation may modulate the activity of ion channels, and what are the major determinants of this regulation. We also discuss how channels act as coincidence detectors sensing phosphoinositide levels and other signalling molecules. We also briefly discuss the available methods to study phosphoinositide regulation of ion channels, and structural aspects of interaction of ion channel proteins with these phospholipids. Finally, in several cases the effect of PtdIns(4,5)P(2) is more complex than a simple dependence of ion channel activity on the lipid, and we will discuss some these complexities.

Transient receptor potential channel 1 (TRPC1) reduces calcium permeability in heteromeric channel complexes

Specific biological roles of the classical transient receptor potential channel 1 (TRPC1) are still largely elusive. To investigate the function of TRPC1 proteins in cell physiology, we studied heterologously expressed TRPC1 channels and found that recombinant TRPC1 subunits do not form functional homomeric channels. Instead, by electrophysiological analysis TRPC1 was shown to form functional heteromeric, receptor-operated channel complexes with TRPC3, -4, -5, -6, and -7 indicating that TRPC1 proteins can co-assemble with all members of the TRPC subfamily. In all TRPC1-containing heteromers, TRPC1 subunits significantly decreased calcium permeation. The exchange of select amino acids in the putative pore-forming region of TRPC1 further reduced calcium permeability, suggesting that TRPC1 subunits contribute to the channel pore. In immortalized immature gonadotropin-releasing hormone neurons endogenously expressing TRPC1, -2, -5, and -6, down-regulation of TRPC1 resulted in increased calcium permeability and elevated basal cytosolic calcium concentrations. We did not observe any involvement of TRPC1 in store-operated cation influx. Notably, TRPC1 suppressed the migration of gonadotropin-releasing hormone neurons without affecting cell proliferation. Conversely, in TRPC1 knockdown neurons, specific migratory properties like distance covered, locomotion speed, and directionality were increased. These findings suggest a novel regulatory mechanism relying on the expression of TRPC1 and the subsequent formation of heteromeric TRPC channel complexes with reduced calcium permeability, thereby fine-tuning neuronal migration.

Baicalein, isolated from Scutellaria baicalensis, protects against endothelin-1-induced pulmonary artery smooth muscle cell proliferation via inhibition of TRPC1 channel expression

ETHNOPHARMACOLOGICAL RELEVANCE

We investigated the antiproliferative effects of baicalein, isolated from Scutellaria baicalensis (Huang-qin), on ET-1-mediated pulmonary artery smooth muscle cells (PASMCs) proliferation and the mechanisms underlying these effects.

MATERIALS AND METHODS

Intrapulmonary artery smooth muscle cells were isolated and cultured from female Sprague-Dawley rats and used during passages 3-6. The proliferation of PASMCs was quantified by cell counting and XTT assay. The protein expression of TRPC1 and PKCα were determined by western blotting. The cell cycle pattern was assayed by flow cytometry. The intracellular calcium concentrations ([Ca(2+)](i)) were measured using the fluorescent indicator fura-2-AM and flow cytometry.

RESULTS

Baicalein (0.3-3 μM) inhibited PASMCs proliferation, promoted cell cycle progression, enhanced [Ca(2+)](i) levels, increased capacitative Ca(2+) entry (CCE), upregulated the canonical transient receptor potential 1 (TRPC1) channel and membrane protein kinase Cα (PKCα) expression induced by ET-1 (0.1 μM). The PKC activator PMA (1 μM) reversed the inhibitory effects of baicalein on ET-1-induced upregulation of TRPC1 expression and S phase accumulation, while the PKC inhibitor chelerythrine (1 μM) potentiated baicalein-mediated G(2)/M phase arrest and TRPC1 channel inhibition.

CONCLUSION

Our findings suggest that baicalein protects against ET-1-induced PASMCs proliferation via modulation of the PKC-mediated TRPC channel.

TRPC1 proteins confer PKC and phosphoinositol activation on native heteromeric TRPC1/C5 channels in vascular smooth muscle: comparative study of wild-type and TRPC1-/- mice

Ca(2+)-permeable cation channels consisting of canonical transient receptor potential 1 (TRPC1) proteins mediate Ca(2+) influx pathways in vascular smooth muscle cells (VSMCs), which regulate physiological and pathological functions. We investigated properties conferred by TRPC1 proteins to native single TRPC channels in acutely isolated mesenteric artery VSMCs from wild-type (WT) and TRPC1-deficient (TRPC1(-/-)) mice using patch-clamp techniques. In WT VSMCs, the intracellular Ca(2+) store-depleting agents cyclopiazonic acid (CPA) and 1,2-bis-(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) both evoked channel currents, which had unitary conductances of ∼2 pS. In TRPC1(-/-) VSMCs, CPA-induced channel currents had 3 subconductance states of 14, 32, and 53 pS. Passive depletion of intracellular Ca(2+) stores activated whole-cell cation currents in WT but not TRPC1(-/-) VSMCs. Differential blocking actions of anti-TRPC antibodies and coimmunoprecipitation studies revealed that CPA induced heteromeric TRPC1/C5 channels in WT VSMCs and TRPC5 channels in TRPC1(-/-) VSMCs. CPA-evoked TRPC1/C5 channel activity was prevented by the protein kinase C (PKC) inhibitor chelerythrine. In addition, the PKC activator phorbol 12,13-dibutyrate (PDBu), a PKC catalytic subunit, and phosphatidylinositol-4,5-bisphosphate (PIP(2)) and phosphatidylinositol-3,4,5-trisphosphate (PIP(3)) activated TRPC1/C5 channel activity, which was prevented by chelerythrine. In contrast, CPA-evoked TRPC5 channel activity was potentiated by chelerythrine, and inhibited by PDBu, PIP(2), and PIP(3). TRPC5 channels in TRPC1(-/-) VSMCs were activated by increasing intracellular Ca(2+) concentrations ([Ca(2+)](i)), whereas increasing [Ca(2+)](i) had no effect in WT VSMCs. We conclude that agents that deplete intracellular Ca(2+) stores activate native heteromeric TRPC1/C5 channels in VSMCs, and that TRPC1 subunits are important in determining unitary conductance and conferring channel activation by PKC, PIP(2), and PIP(3).

Enhanced store-operated Ca²+ entry and TRPC channel expression in pulmonary arteries of monocrotaline-induced pulmonary hypertensive rats

Pulmonary hypertension (PH) is associated with profound vascular remodeling and alterations in Ca(2+) homeostasis in pulmonary arterial smooth muscle cells (PASMCs). Previous studies show that canonical transient receptor potential (TRPC) genes are upregulated and store-operated Ca(2+) entry (SOCE) is augmented in PASMCs of chronic hypoxic rats and patients of pulmonary arterial hypertension (PAH). Here we further examine the involvement of TRPC and SOCE in PH with a widely used rat model of monocrotaline (MCT)-induced PAH. Rats developed severe PAH, right ventricular hypertrophy, and significant increase in store-operated TRPC1 and TRPC4 mRNA and protein in endothelium-denuded pulmonary arteries (PAs) 3 wk after MCT injection. Contraction of PA and Ca(2+) influx in PASMC evoked by store depletion using cyclopiazonic acid (CPA) were enhanced dramatically, consistent with augmented SOCE in the MCT-treated group. The time course of increase in CPA-induced contraction corresponded to that of TRPC1 expression. Endothelin-1 (ET-1)-induced vasoconstriction was also potentiated in PAs of MCT-treated rats. The response was partially inhibited by SOCE blockers, including Gd(3+), La(3+), and SKF-96365, as well as the general TRPC inhibitor BTP-2, suggesting that TRPC-dependent SOCE was involved. Moreover, the ET-1-induced contraction and Ca(2+) response in the MCT group were more susceptible to the inhibition caused by the various SOCE blockers. Hence, our study shows that MCT-induced PAH is associated with increased TRPC expression and SOCE, which are involved in the enhanced vascular reactivity to ET-1, and support the hypothesis that TRPC-dependent SOCE is an important pathway for the development of PH.

Proteomic analyses of human plasma: Venus versus Mars

BACKGROUND

Plasma is vital for the resuscitation of injured patients and to restore necessary procoagulants, especially Factors (F)II, FV, FVII, FX, and FXIII; however, female plasma has been implicated in the majority of transfusion-related acute lung injury (TRALI) cases and male-only plasma transfusion regimens have significantly decreased the incidence of TRALI. Little is known about the human plasma proteome, and no comparisons have been made between male and female plasma; therefore, we hypothesize that there are significant differences between plasma from male and female donors.

STUDY DESIGN AND METHODS

Five units of fresh-frozen plasma each were collected from nulliparous female donors and male donors, and the proteome was analyzed by depleting the 14 most common proteins by immunoaffinity columns followed by protein separation by one dimension gel electrophoresis, tryptic digestion of the proteins, analysis of the peptides by liquid chromatography-tandem mass spectrometry, and identification employing human protein sequence databases.

RESULTS

Female plasma versus male plasma contained pregnancy zone protein (419- to 580-fold), FV (twofold), α(1)-antitrypsin (twofold), β(2) -microglobulin (twofold), and Complement Factors H and C4B (1.5- to 2-fold) at significantly higher concentrations than males and males contained significant increases in Fc-binding protein (twofold), protein Z-dependent protease inhibitor (twofold), phosphatidylinositol glycan-specific phospholipase (fourfold), protein S-100 (threefold), and transgelin-2 (14-fold) versus females (p < 0.005). The increases in FV, α(1)-antitrypsin, and β(2)-microglobulin were confirmed by an activity assay or immunoblots.

CONCLUSION

We conclude that there are proteomic differences between male and female plasma, which could be exploited to improve clinical outcomes in transfused patients.

Three 4-letter words of hypertension-related cardiac hypertrophy: TRPC, mTOR, and HDAC

Left ventricular hypertrophy due to hypertension represents a major risk factor for adverse cardiovascular events and death. In recent years, the prevalence of cardiac hypertrophy has increased due to obesity and an aging population. Notably, a significant number of individuals have persistent cardiac hypertrophy in the face of blood pressure that is normalized by drug treatment. Thus, a better understanding of the processes underlying the cardiac remodeling events that are set into play by hypertension is needed. At the level of the cardiac myocytes, hypertrophic growth is often described as physiological, as occurs with exercise, or pathological, as seen with hypertension. Here we discuss recent developments in three areas that are fundamental to pathological hypertrophic growth of cardiac myocytes. These areas are the transient receptor potential canonical (TRPC) channels, mammalian target of rapamycin (mTOR) complexes, and histone deacetylase (HDAC) enzymes. In the last several years, studies in each of these areas have yielded new and exciting discoveries into the genesis of pathological growth of cardiac myocytes. The phosphoinositide 3-kinase-Akt signaling network may be the common denominator that links these areas together. Defining the interrelationship among TRPC channels, mTOR signaling, and HDAC enzymes is a promising, but challenging area of research. Such knowledge will undoubtedly lead to new drugs that better prevent or reverse left ventricular hypertension.

Protein kinases modulate store-operated channels in pulmonary artery smooth muscle cells

Background

This study investigates whether protein kinase G (PKG), protein kinase A (PKA) and protein kinase C (PKC) are involved in the regulatory mechanisms of store-operated channel (SOC) in pulmonary arteries.

Methods

Pulmonary artery smooth muscle cells (PASMCs) were enzymatically dissociated from rat intralobar pulmonary arteries. Whole cell, cell-attached and inside-out patch-clamp electrophysiology were used to monitor SOCs in isolated PASMCs.

Results

Initially the Ca²⁺-ATPase inhibitor cyclopiazonic acid (CPA, 10 μM) initiated a whole cell current that was reduced by the SOC blocker SKF-96365 (10 μM). Subsequent work using both cell-attached and whole cell configurations revealed that the PKG and PKA inhibitors, KT5823 (3 μM) and H-89 (10 μM), also stimulated SOC activity; this augmentation was attenuated by the SOC blockers SKF-96365 (10 μM) and Ni²⁺ (0.1 mM). Finally using the inside-out configuration, the PKC activator phorbol 12-myristate 13-acetate (PMA, 10 μM) was confirmed to modestly stimulate SOC activity although this augmentation appeared to be more substantial following the application of 10 μM inositol 1,4,5-triphosphate (Ins(1,4,5)P₃).

Conclusions

SOC activity in PASMCs was stimulated by the inhibition of PKG and PKA and the activation of PKC. Our findings suggest that the SOC could be a substrate of these protein kinases, which therefore would regulate the intracellular concentration of calcium and pulmonary arteriopathy via SOC.

Chemotaxis of MDCK-F cells toward fibroblast growth factor-2 depends on transient receptor potential canonical channel 1

Movement toward the source of a chemoattractant gradient is a basic cellular property in health and disease. Enhanced migration during metastasis involves deregulated growth factor signaling. Growth factor stimulation and cell migration converge both on the important second messenger Ca(2+). To date, the molecular identification of Ca(2+) entry pathways activated by growth factors during chemotaxis is still an open issue. We investigated the involvement of the nonselective Ca(2+) channel TRPC1 (transient receptor potential canonical 1) in FGF-2 guided chemotaxis by means of time-lapse video microscopy and by functional Ca(2+) measurements. To specifically address TRPC1 function in transformed MDCK cells we altered the expression levels by siRNA or overexpression. We report that TRPC1 channels are required for the orientation of transformed MDCK cells in FGF-2 gradients because TRPC1 knockdown or pharmacological blockade prevented chemotaxis. Stimulation with FGF-2 triggered an immediate Ca(2+) influx via TRPC1 channels that depended on phospholipase C and phosphatidylinositol 3-kinase signaling. Impeding this Ca(2+) influx abolished chemotaxis toward FGF-2. This functional connection correlated with clustering of FGF receptors and TRPC1 channels as was observed by immunolabeling. These findings show the important interplay between growth factor signaling and Ca(2+) influx in chemotaxis.

TRPC6 channels stimulated by angiotensin II are inhibited by TRPC1/C5 channel activity through a Ca2+- and PKC-dependent mechanism in native vascular myocytes

The present work investigated interactions between TRPC1/C5 and TRPC6 cation channel activities evoked by angiotensin II (Ang II) in native rabbit mesenteric artery vascular smooth muscle cells (VSMCs). In low intracellular Ca(2+) buffering conditions (0.1 mm BAPTA), 1 nm and 10 nm Ang II activated both 2 pS TRPC1/C5 channels and 15-45 pS TRPC6 channels in the same outside-out patches. However, increasing Ang II to 100 nm abolished TRPC6 activity but further increased TRPC1/C5 channel activity. Comparison of individual patches revealed an inverse relationship between TRPC1/C5 and TRPC6 channel activity suggesting that TRPC1/C5 inhibits TRPC6 channel activity. Inclusion of anti-TRPC1 and anti-TRPC5 antibodies, raised against intracellular epitopes, in the patch pipette solution blocked TRPC1/C5 channel currents but potentiated by about six-fold TRPC6 channel activity evoked by 1-100 nm Ang II in outside-out patches. Bath application of T1E3, an anti-TRPC1 antibody raised against an extracellular epitope, also increased Ang II-evoked TRPC6 channel activity. With high intracellular Ca(2+) buffering conditions (10 mm BAPTA), 10 nm Ang II-induced TRPC6 channel activity was increased by about five-fold compared to channel activity with low Ca(2+) buffering. In addition, increasing intracellular Ca(2+) levels ([Ca(2+)](i)) at the cytosolic surface inhibited 10 nm Ang II-evoked TRPC6 channel activity in inside-out patches. Moreover, in zero external Ca(2+) (0 [Ca(2+)](o)) 100 nm Ang II induced TRPC6 channel activity in outside-out patches. Pre-treatment with the PKC inhibitor, chelerythrine, markedly increased TRPC6 channel activity evoked by 1-100 nm Ang II and blocked the inhibitory action of [Ca(2+)](i) on TRPC6 channel activity. Co-immunoprecipitation studies shows that Ang II increased phosphorylation of TRPC6 proteins which was inhibited by chelerythrine, 0 [Ca(2+)](o) and the anti-TRPC1 antibody T1E3. These results show that TRPC6 channels evoked by Ang II are inhibited by TRPC1/C5-mediated Ca(2+) influx and stimulation of PKC, which phosphorylates TRPC6 subunits. These conclusions represent a novel interaction between two distinct vasoconstrictor-activated TRPC channels expressed in the same native VSMCs.

Channelopathies linked to plasma membrane phosphoinositides

The plasma membrane phosphoinositide phosphatidylinositol 4,5-bisphosphate (PIP2) controls the activity of most ion channels tested thus far through direct electrostatic interactions. Mutations in channel proteins that change their apparent affinity to PIP2 can lead to channelopathies. Given the fundamental role that membrane phosphoinositides play in regulating channel activity, it is surprising that only a small number of channelopathies have been linked to phosphoinositides. This review proposes that for channels whose activity is PIP2-dependent and for which mutations can lead to channelopathies, the possibility that the mutations alter channel-PIP2 interactions ought to be tested. Similarly, diseases that are linked to disorders of the phosphoinositide pathway result in altered PIP2 levels. In such cases, it is proposed that the possibility for a concomitant dysregulation of channel activity also ought to be tested. The ever-growing list of ion channels whose activity depends on interactions with PIP2 promises to provide a mechanism by which defects on either the channel protein or the phosphoinositide levels can lead to disease.

Ins(1,4,5)P3 interacts with PIP2 to regulate activation of TRPC6/C7 channels by diacylglycerol in native vascular myocytes

We investigated synergism between inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and diacylglycerol (DAG) on TRPC6-like channel activity in rabbit portal vein myocytes using single channel recording and immunoprecipitation techniques. Ins(1,4,5)P(3) at 10 microm increased 3-fold TRPC6-like activity induced by 10 microm 1-oleoyl-2-acetyl-sn-glycerol (OAG), a DAG analogue. Ins(1,4,5)P(3) had no effect on OAG-induced TRPC6 activity in mesenteric artery myocytes. Anti-TRPC6 and anti-TRPC7 antibodies blocked channel activity in portal vein but only anti-TRPC6 inhibited activity in mesenteric artery. TRPC6 and TRPC7 proteins strongly associated in portal vein but only weakly associated in mesenteric artery tissue lysates. Therefore in portal vein the conductance consists of TRPC6/C7 subunits, while OAG activates a homomeric TRPC6 channel in mesenteric artery myocytes. Wortmannin at 20 microm reduced phosphatidylinositol 4,5-bisphosphate (PIP(2)) association with TRPC6 and TRPC7, and produced a 40-fold increase in OAG-induced TRPC6/C7 activity. Anti-PIP(2) antibodies evoked TRPC6/C7 activity, which was blocked by U73122, a phospholipase C inhibitor. DiC8-PIP(2), a water-soluble PIP(2) analogue, inhibited OAG-induced TRPC6/C7 activity with an IC(50) of 0.74 microm. Ins(1,4,5)P(3) rescued OAG-induced TRPC6/C7 activity from inhibition by diC8-PIP(2) in portal vein myocytes, and this was not prevented by the Ins(1,4,5)P(3) receptor antagonist heparin. In contrast, Ins(1,4,5)P(3) did not overcome diC8-PIP(2)-induced inhibition of TRPC6 activity in mesenteric artery myocytes. 2,3,6-Tri-O-butyryl-Ins(1,4,5)P(3)/AM (6-Ins(1,4,5)P(3)), a cell-permeant analogue of Ins(1,4,5)P(3), at 10 microm increased TRPC6/C7 activity in portal vein and reduced association between TRPC7 and PIP(2), but not TRPC6 and PIP(2). In contrast, 10 microm OAG reduced association between TRPC6 and PIP(2), but not between TRPC7 and PIP(2). The present work provides the first evidence that Ins(1,4,5)P(3) modulates native TRPC channel activity through removal of the inhibitory action of PIP(2) from TRPC7 subunits.